public extension PureXML.Validation.DTDSchema {
    /// Returns `node` with DTD attribute defaults applied: every element gains any
    /// attribute its `<!ATTLIST>` declares with a default or `#FIXED` value and
    /// that the element omits. `#REQUIRED` and `#IMPLIED` add nothing. The injected
    /// values are the normalized defaults from the declaration, so an editor reads
    /// the same effective attributes a validating processor would see.
    func applyingDefaults(to node: PureXML.Model.Node) -> PureXML.Model.Node {
        switch node {
        case let .document(children):
            .document(children.map(applyingDefaults(to:)))
        case let .element(element):
            .element(applyingDefaults(to: element))
        case .text, .cdata, .comment, .processingInstruction:
            node
        }
    }

    private func applyingDefaults(to element: PureXML.Model.Element) -> PureXML.Model.Element {
        var result = element
        result.children = element.children.map { applyingDefaults(to: $0) }
        guard let declarations = attributes[element.name.description] else { return result }
        for declaration in declarations {
            guard let value = Self.defaultValue(declaration) else { continue }
            let present = result.attributes.contains {
                $0.name.description == declaration.name || $0.name.localName == declaration.name
            }
            if !present { result.attributes.append(PureXML.Model.Attribute(declaration.name, value)) }
        }
        return result
    }

    private static func defaultValue(_ declaration: PureXML.Validation.AttributeDeclaration) -> String? {
        switch declaration.defaultDecl {
        case let .value(value), let .fixed(value): value
        case .required, .implied: nil
        }
    }
}